This invention relates to a novel method for preparing inorganic sulfides of calcium, strontium, and combinations of calcium and strontium. The sulfides may be luminescent or nonluminescent.
It is known that phosphors in the calcium-sulfide and strontium-sulfide families have potential for use in practical applications involving photoluminescence, cathodoluminescence and electroluminescence because of the variety of emission colors and the relatively high efficiencies that are obtainable from these materials in laboratory-made devices. However, phosphors in these families are at least partially soluble in water, and their luminescence is degraded when they are exposed to water or even high humidity. Thus, these phosphors must be protected against exposure to moisture throughout the manufacture and operation of any device in which they are used.
Calcium and/or strontium sulfides are usually prepared by the sulfurization of the corresponding oxides or carbonates by heating in a hydrogen sulfide or carbon disulfide atmosphere. Also, these sulfides may be prepared by chemically reducing the corresponding sulfates with hydrogen or carbon, or by reacting the corresponding oxides or carbonates with sulfur. The sulfide products prepared by any of these methods consist of very small particles (with relatively large surface areas) that are unstable in humid atmospheres or when contacted with water.
Calcium sulfides may also be prepared by chemically reducing calcium sulfate with carbon particles in the presence of a large amount of sodium sulfate, which acts as a flux for crystallizing the product. The calcium sulfide prepared by this carbon-flux method consists of relatively larger grains and is comparatively stable in humid atmospheres or when contacted with water. However, phosphors that are prepared by this method have relatively poor luminescence efficiencies, at least partially for the reason that some residual carbon particles remain in the product after all attempts at carbon removal have been completed.
A sulfurizing-flux method has been used previously to synthesize inorganic oxysulfides; for example, by heating a mixture of an inorganic oxide with a large amount of a reactive sulfurizing flux, such as sodium thiosulfate, or sodium carbonate and sulfur. The mixture is heated until the flux melts and reacts with the inorganic oxide and the mixture forms into a molten mass. The molten mass is cooled to room temperature, and crystallized particles of the desired inorganic oxysulfide, which are water-insoluble, are recovered by dissolving away the residual hardened mass, which is water-soluble. This method, which is used to prepare water-insoluble materials, has not been suggested for use in synthesizing calcium and/or strontium sulfides for at least two reasons. First, the method is known to produce oxysulfides, not sulfides. Second, the method requires a water-washing step in order to dissolve away the residual hardened flux. Calcium and strontium sulfides, as previously prepared, are too water-soluble and too water-sensitive to tolerate the required water-washing step.